The present invention relates to a multi-beam generating element and an optical printing apparatus to be used therewith, and more particularly to a multi-beam generating element and an optical printing apparatus which a laser beam is branched into a plurality of beams, which are processed in parallel to realize a high speed processing.
A laser beam incident upon a grating element is branched into multiple diffracted light beams. There have been a lot of proposals which use this phenomenon is as a multi-beam generating element, and there have been some applications developed as well. However, it has been difficult to increase the number of branched beams and to efficiently make the intensity distribution of the branched beams uniform.
As a countermeasure against this problem, a method which uses a grating element for converting a single beam into multiple beams has been proposed in Technical Digest on The 3rd Microoptics Conference, page 84 (1991). The method, however, includes a complicated fabricating process where the intensity of an electron beam needs to be accurately controlled using many intensity levels to irradiate a material to be exposed by the electron beam since the material is formed to have a multi-level phase pattern. Further, there are limitations in the number of the branched beams, the efficiency of light utilization and the uniformity in the light intensity distribution of the branched beams, since the number of levels in the multi-level phase pattern is limited due to a limitation in the fabrication process.
The inventors have proposed a grating element for converting a single beam into multiple beams where the basic phase patterns composing the grating are formed in fractionized rectangular patterns having non-uniform widths and have the phase height of a single level. In this method, by optimizing and determining the non-uniform widths in the rectangular patterns, it is possible to obtain an odd number of multiple beams having an excellent uniformity and a high efficiency of light utilization. The grating according to this method can be easily fabricated, since the pattern is formed of a single layer coating material.
The method according to the above invention, however, cannot generate an even number of multiple beams. A electronic control circuit used together with an optical apparatus is constructed on the basis of information composed of 1 byte or 8 bits, that is, an even number. Further, a flip-flop circuit and/or a counter circuit frequently used in a control circuit have a basic operation for converting the number of electric pulse signals into twice or a half as many as the original electric pulse signals. For these reasons, it is frequently required that the number of multiple beams used in an optical apparatus is an even number.
On the other hand, it is well known that scanning by use of a plurality of laser beams is an effective method for a high speed laser printer. For example, in a laser printer which the inventors have proposed, a plurality of laser beams generated by a grating are converted into parallel multiple beams using a lens and are inputted to a multi-channel acousto-optical modulator. Then, a plurality of the laser beams modulated by the multi-channel acousto-optical modulator are scanned on a photosensitive drum using a rotating polygonal mirror.
To form a beam spot array at an oblique angle on the photosensitive drum in the optical system described above, the grating and the multi-channel acousto-optical modulator need to be accurately adjusted so as to have a tilt at a given angle to a horizontal plane. It is theoretically possible to adjust the two units together, but practically it has been extremely difficult. Especially for the multi-channel acousto-optical modulator, in order to efficiently diffract the primary degree diffracted beam among all of the incident multiple beams, the angle adjustment in the plane diffracting the beams, that is, the adjustment in that Bragg angle, requires an extremely high accuracy. In addition to this, it has been very difficult to effect an adjustment to form the beam spot array an oblique angle on the photosensitive drum.